A152141 a(n) is the least k such that 3*2^n*(2^k-1)-1 or 3*2^n*(2^k-1)+1 is prime ( or both primes).

1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 1, 1, 2, 2, 2, 3, 2, 1, 5, 7, 2, 4, 4, 4, 8, 9, 3, 4, 4, 1, 11, 13, 2, 1, 9, 1, 3, 1, 5, 8, 1, 2, 1, 5, 6, 20, 15, 6, 7, 8, 3, 5, 13, 4, 1, 6, 43, 8, 4, 4, 7, 9, 2, 1, 2, 1, 2, 15, 42, 5, 10, 8, 18, 3, 10, 1, 8, 8, 7, 21, 2, 16, 19, 5, 4

Offset

0,10

Links

Pierre CAMI, Table of n, a(n) for n = 0..1600

Example

3*2^0*(2^1-1)-1=2 is prime so a(0)=1;
3*2^1*(2^1-1)-1=5 is prime as well as 7 so a(1)=1;
3*2^2*(2^1-1)-1=23 is prime so a(2)=1.

Prog

(PARI) a(n) = my(k=1); while ((x=3*2^n*(2^k-1)) && !isprime(x-1) && !isprime(x+1), k++); k; \\ Michel Marcus, Sep 16 2019; corrected Jun 14 2022

Crossrefs

Sequence in context: A367825 A107333 A161642 * A098505 A178395 A330958

Adjacent sequences: A152138 A152139 A152140 * A152142 A152143 A152144

Keyword

nonn

Author

Pierre CAMI, Nov 26 2008

Status

approved